The present invention generally relates to fluid heaters. More specifically, the present invention relates to parenteral fluid heaters and methods of heating parenteral fluids. In an embodiment, the present invention relates to fluid heaters for use in dialysis systems.
Due to disease, insult or other causes, a person's renal system can fail. In renal failure of any cause, there are several physiological derangements. The balance of water, minerals and the excretion of daily metabolic load is no longer possible in renal failure. During renal failure, toxic end products of nitrogen metabolism (urea, creatinine, uric acid, and others) can accumulate in blood and tissues.
Kidney failure and reduced kidney function have been treated with dialysis. Dialysis removes waste, toxins and excess water from the body that would otherwise have been removed by normal functioning kidneys. Dialysis treatment for replacement of kidney functions is critical to many people because the treatment is life saving. One who has failed kidneys could not continue to live without replacing at least the filtration functions of the kidneys. Hemodialysis and peritoneal dialysis are two types of dialysis therapies commonly used to treat loss of kidney function.
Peritoneal dialysis utilizes a dialysis solution or dialysate, which is infused into a patient's peritoneal cavity. The dialysate contacts the patient's peritoneal membrane in the peritoneal cavity. Waste, toxins, and excess water pass from the patient's bloodstream through the peritoneal membrane and into the dialysate. The transfer of waste, toxins, and water from the bloodstream into the dialysate occurs by diffusion and osmosis because there is an osmotic gradient across the peritoneal membrane. The spent dialysate is drained from the patient's peritoneal cavity to remove the waste, toxins, and water from the patient and then replaced.
Prior to infusion into the peritoneal cavity, the dialysis solution is frequently at a temperature lower than body temperature. For example, the dialysis solution may be at room temperature or even colder. Dialysis solution can be particularly cold when stored in a cold place or exposed to cold weather. Using dialysate that is cold relative to the patient undesirably cools the patient and can cause the patient discomfort during the dialysis procedure. Accordingly, it is desirable to heat the dialysate to about body temperature prior to infusion into the patient's peritoneal cavity.
Hemodialysis treatment utilizes the patient's blood to remove waste, toxins and excess water from the patient. The patient is connected to a hemodialysis machine and the patient's blood is pumped through the machine. Catheters are inserted into the patient's veins and arteries to connect the blood flow to and from the hemodialysis machine. As blood passes through a dialyzer in the hemodialysis machine, the dialyzer removes the waste, toxins and excess water from the patient's blood and returns the blood to infuse back into the patient. It is also desirable to heat fluids, for example blood and dialysate, used in hemodialysis to about body temperature.
Heating dialysis fluids to temperatures comfortable to the patient has historically been accomplished using electrical resistive plate heaters. Resistive plate heaters increase in temperature when electricity is applied to the resistive plate. The resistive plate heaters are in direct contact with a fluid container and transfer heat from the plate to the fluid in the container.
Two types of existing resistive plate heaters include a bulk plate heater and an in-line plate heater. A bulk plate heater can have a relatively large dialysate container, such as a two liter dialysate reservoir bag, placed on top of the heater plate. The bulk plate heater heats the fluid close to the heater plate in the reservoir bag, and over time the temperature increase spreads throughout the fluid in the bag to heat all of the dialysate fluid. In-line plate heaters heat dialysate fluid as the fluid flows through a relatively smaller bag in contact with the heater plate. In-line plate heaters purport to heat fluid on demand as the fluid flows past the heater plate, whereas bulk plate heaters provide a reservoir of heated fluid.
Resistive plate heaters have also been used for heating fluids during hemodialysis and intravenous administration of fluids. Although resistive plate heaters have been used to heat fluids for dialysis treatments and other applications, resistive plate heaters have limitations. For example, the heating capacity of a plate heater is dependant on the surface area of the heater plate. If larger dialysate loads need to be heated, the size of the surface area of the heater plate must be increased. Increasing the size of the heater may not be desired for various reasons, such as requiring more space or higher electrical power consumption. Alternatively, the temperature of the heater plate can be increased; however, higher temperatures may not be desired. For example, the temperature of the heater plate may be limited to safe levels for the fluid being heated or to safe levels for the fluid container in contact with the heater plate.
Another limitation with resistive plate heaters is that the heaters are two-dimensional, i.e., the heaters transfer heat energy over the two-dimensional surface area of the heater plate. Temperature increase of the fluid being heated occurs at the interface between the fluid and the fluid container, i.e, sheeting which contacts the heater plate. Accordingly, the plate heater only directly heats one surface of the fluid. The remaining fluid is heated by heat conduction and convection from the heated surface. The two-dimensional heating of the dialysate fluid limits the depth of a dialysate container in a bulk heating operation and the fluid flow rate in an in-line operation. Further, the wall of the dialysate container in either type of operation necessarily lowers the heating response time of the system in comparison to a direct contact of the fluid with a heat source. A flexible plastic dialysate bag material typically does not have good thermal conductivity. Known bulk and in-line heaters can consequently have a slow response time when a relatively large dialysate load needs to be heated.
Another problem with known bulk and in-line heaters is that the dialysate bag or container, which is a necessary component in either system, gives off heat as the bulk or in-line heat plate attempts to heat the dialysate fluid. The heat plate heats one side of the container, while the remaining sides of the container give off heat to the atmosphere due to convective or evaporative cooling.
A further problem with the present bulk and in-line dialysate heaters is that the heat plates remain hot for a period of time after being turned off. That is, the stored thermal energy of the heated plates does not immediately dissipate when the electricity powering the plates is cut off. The result is that either the system throws away the additional heat or shuts down the heater prior to the time that the fluid reaches its desired temperature in an attempt to use the residual heat to heat the fluid to its desired temperature. The first option creates inefficiency, while the second option increases complexity and chance for error. Generally, accurate control of the fluid temperature is difficult with bulk fluid heaters.
Accordingly, a need exists to provide a more efficient dialysate heater, which is capable of heating relatively large dialysate loads and which has an improved turn-off response time.